1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines, facsimile machines, and printers and, more particularly, to image forming apparatuses that include a toner supply container that is used to mix new toner and old toner.
2. Description of the Related Art
In electrophotographic image forming apparatuses, a drum-shaped photosensitive element is evenly charged using a charging roller and then a latent image is formed on the photosensitive element by scanning it with a laser beam. The latent image is then developed into a visible image with toner accommodated in a developing device.
A recording medium or a transfer sheet receives the visible image at the position where the photosensitive element comes into contact with a transferring roller, and the toner of the image is melted and fixed to the transfer sheet by a fixing device.
The developing device includes a developing roller that serves as a developer carrier; a supplying roller that supplies non-magnetic monocomponent toner (negatively charged) to the developing roller; a stirring member that conveys, within a container, toner to be closer to the supplying roller; and a developing blade that serves as a developer adjusting member and adjusts the amount of toner on the developing roller. Because the developing roller comes into contact with the photosensitive element, the developing roller is made of an elastic material. In most cases, the developing blade is brought into press-contact by the spring force of a sheet of metal. During development, in order to transfer toner from the developing roller to the photosensitive element, the developing roller is charged to a predetermined potential by a developing-bias power supply.
The developing blade of the developing device is connected to a blade-bias power supply so that the toner maintains a certain amount of charge. There are various types of blade-bias power supplies, for example, a blade-bias power supply that charges to the same potential as the developing-bias power supply and a blade-bias power supply that charges to a different potential.
For example, in one case, the developing roller always has a negative charge of 300 volts from the developing-bias power supply and the developing blade always has a negative charge of 400 volts from the blade-bias power supply so that there is a potential difference of about 100 volts between the developing roller and the developing blade. This allows the toner to maintain a certain amount of charge and is effective in preventing the leakage of toner. In another case, an AC bias is applied constantly to the developing blade.
When such an image forming apparatus prints out a large number of copies, toner is consumed during the printing. Hence, it is necessary to supply toner after such a large amount of printing. The toner supply involves, in general, removing a sealing member from a toner supply container that contains toner, opening a lid of the developing device, and supplying the toner to the developing device of an electrostatic recording device.
Mutual charging occurs between the new toner that is newly supplied to the developing device and the old toner that has been used by the developing device. As a result, the charge of the new toner increases, while the charge of the old toner, which is present in the developing device before the toner supply, decreases or even goes toward the opposite polarity. This leads to a malfunction such as increased scumming.
The reason why the mutual charging occurs is described below. The new toner in the developing device is subjected to stress, during the printing operation, when passing through the supplying roller, the developing roller, the developing blade, and the photosensitive element. Due to the stress, an external additive agent is removed from particles of the toner or embedded inside the particles from the surface. Moreover, some particles of the toner are modified or broken into pieces. Eventually, the electric potential of the old toner decreases. When the old toner is mixed with the new toner, i.e., the two toners having different electric potentials are mixed with each other, electric interaction occurs between the different toners. As a result, the charge of one toner increases relative to the charge before mixing and the charge of the other toner decreases relative to the usual charge.
In actual image formation, a technology is needed for suppressing the mutual charging that occurs within the developing device, thereby maintaining the image quality at a high level with no scumming.
Utility Model Application No. 6-69960 discloses a toner supplying device that is used in a recording device. The toner supplying device includes a toner supply container that has an opening and contains toner therein. The toner supplying device supplies the toner from the toner supply container to a developing device. The toner supplying device includes a toner supply port from which the toner is supplied; a supporting member that supports the toner supply container so that the opening fits with the toner supply port; and a vibration dampener that is provided on the supporting member. The toner supplying device supplies the toner by vibrating the toner supply container. Utility Model Application No. 6-69960 discloses a toner supplying device that can supply toner from the toner supply container to the developing device in an efficient manner but it does not state that used old toner is mixed with unused new toner in the toner supply container.
Japanese Patent No. 3459860 discloses a developing device that is arranged on a substantially horizontal plane and provided with a toner supply container. The toner supply container rotates around the axis thereof to supply toner. When the container becomes empty after toner supply, degraded developer is conveyed to the container for developer collection. The container has an opening with a cap. The opening position of the cap during the toner supply is different from the opening position of the cap during the developer collection. Japanese Patent No. 3459860 discloses technology related to the shape of the toner supply container but it does not state that used old toner is mixed with unused new toner in the toner supply container.